TM 1-1510-218-10 3B-1 CHAPTER 3B AVIONICS  D2   Section I.  GENERAL 3B-1.  GENERAL DESCRIPTION. This    chapter    covers    the    avionics    equipment configuration installed in C-12D2 aircraft.  It includes a brief     description     of     the     avionics     equipment,     its technical characteristics, capabilities, and locations. 3B-2.  AVIONICS EQUIPMENT CONFIGURATION. The    avionics    configuration    of    the    aircraft    is comprised   of   three   groups   of   electronic   equipment.   The  communication  equipment  group  consists  of  the interphone,  UHF  command,  VHF  command,  and  HF command systems.  The  navigation equipment group provides  the  pilot  and  copilot  with  the  instrumentation required  to  establish  and  maintain  an  accurate  flight course   and   position   and   to   make   an   approach   on instruments under Instrument Meteorological Conditions   (IMC).      The   navigation   group   includes equipment  for  determining  altitude,  attitude,  position, destination,   range   and   bearing,   heading   reference, groundspeed,  and  drift  angle.    The  transponder  and radar    group    includes    identification,    position,    and emergency  tracking  system    and   a   radar   system   to locate potentially dangerous weather areas.  A Ground Proximity  Altitude  Advisory  System  (GPAAS)  is  also installed. 3B-3.  POWER SOURCE. a.     DC   Power.      DC   power   for   the   avionics equipment   is   provided   by   four   sources:   the   aircraft battery,   the   left   and   right   generators,   and   external power.  Power is routed through two 50-ampere circuit breakers to the avionics power relay that is controlled by the  AVIONICS MASTER POWER SWITCH on the overhead control panel.  Refer to Figure 2-15, Sheet 1.   Individual  system  circuit  breakers  and  the  associated avionics     buses     are     shown     in     Figure 2-28     and Table 2-7.    With  the  switch  in  the   ON  position,  the avionics   power   relay   is   de-energized   and   power   is applied  through  both  50-ampere  AVIONICS MASTER POWER  #1 and   #2  circuit  breakers  to  the  individual avionics    circuit    breakers    on    the    overhead    circuit breaker panel.  Refer to Figure 2-16, Sheet 3. NOTE Should   the   AVIONICS   MASTER   POWER switch     fail     to     operate,     power     to     the individual  avionics  circuit  breakers  can  be provided   by   pulling   the   5-ampere  circuit breaker,     placarded     AVIONICS     MASTER CONTROL, located on the overhead circuit breaker panel. In the off (aft) position, the relay is energized and power  is  removed  from  avionics  equipment.    When external  power  is  applied  to  the  aircraft,  the  avionics power  relay  is  normally  energized,  removing  power from the avionics equipment.  To apply external power to    the    avionics    equipment,    move    the    AVIONICS MASTER  POWER  switch  to  the   EXT  PWR position.   This   will   de-energize   the   avionics  power  relay  and allow power to be applied to avionics equipment. b.    AC  Power.  Two  750-volt  ampere  inverters provide  ac  power  for  the  avionics  equipment.    The inverters  supply  115-volt  and  25-volt  single-phase  ac power   when   operated   by   the   INVERTER   #1   or   #2 switches.      Normally   both   switches   will   be   ON   for operation.  However, either inverter will provide power for  all  avionics  equipment  requiring  ac  power,  should one inverter fail.  115 Vac power from the inverters is routed   through   fuses   and   transformers   in   the   nose avionics  compartment.    The  transformers  provide  the required 26 Vac needed by avionics equipment. Section II.  COMMUNICATIONS 3B-4.  DESCRIPTION. The  communications  equipment  group  consists of the interphone, UHF command, VHF command, and HF command systems. 3B-5.  MICROPHONE     SWITCHES,     MICROPHONE JACKS, AND HEADSET JACKS. For information on microphone switches, microphone     jacks,     and     headset     jacks,     refer     to Chapter  3, Paragraph 3-5.